Having a cyclist take a breath test is good for evidence at court. I remember giving a breath test to a van driver. I could smell the alcohol on him but he seemed totally lucid, something just did not add up. At the police station he was performing card tricks and coin tricks and was very good.
He was 4 times over the limit and I never did work out how he appeared to be as sober as he appeared.
My record result is 139 on the station procedure. He was one of the 'more sober' (in appearance) people I've dealt with too. I remember my colleague at the time saying "I wouldn't have even breathalysed him!" as he had noticed anything strange about him. I thought I could smell a little alcohol so did...
It's to do with your regular intake. An alcoholic drinking daily will show less effects for the alcohol than a teetotaller. You may argue that makes an alcoholic a safer driver on a few pints than a normal drinker, and there may even be a point there - but as a poster above pointed out, (fortunately) there are set arbitrary limits rather than having to argue whether people are unfit or not.
For the record, I have breathalysed cyclists, but have always pointed out it was not a legal requirement, indeed the reason I offer it is in case the other driver later tries to muddy the water by saying "I was breathalysed and they weren't, they were probably drunk". So far the cyclists have always taken up the offer, and have anyways blown zero.